Activated carbon is a form of carbon thatis shot through with nanosized holes that increase the material's surface areaand allow it to catalyze more chemical reactions and store more electricalcharge. But due to the way it is produced, most of the pores within it aren'tinterconnected, limiting the material's ability to transport electricity. Nowresearchers at Stanford University have created a "designer carbon"with greater pore connectivity and therefore greater electronicconductivity, which enables superior energy-storage performance.
Used for everything from gas and waterpurification to air filters and medicine, activated carbon is generallyproduced from coconut husks, nutshells, peat, wood and other carbon-rich sourcematerials. These are burnt at high temperatures and then subjected to eitherphysical (aka steam) activation or chemical activation to create nanosizedpores. But the random nature of these processes results in a material withlittle interconnectivity between the pores.
"With activated carbon,there's no way to control pore connectivity," says Zhenan Bao, the seniorauthor of the study and a professor of chemical engineering atStanford. "Also, lots of impurities from the coconut shells and otherraw starting materials get carried into the carbon. As a refrigeratordeodorant, conventional activated carbon is fine, but it doesn't provide highenough performance for electronic devices and energy-storageapplications."
To create their new high quality designer carbon, Bao and her colleagues started with a conducting,water-based polymer known as a hydrogel.
"Hydrogel polymers form aninterconnected, three-dimensional framework that's ideal for conductingelectricity," Bao said. "This framework also contains organicmolecules and functional atoms, such as nitrogen, which allow us to tune theelectronic properties of the carbon."
Using a mild carbonization and activationprocess, the Stanford team converted the hydrogel into nanometer-thick sheetsof carbon with a 3D network boasting higher pore connectivity. To activate thecarbon sheets and increase their surface area, they then added potassiumhydroxide.
"We call it designer carbonbecause we can control its chemical composition, pore size and surface areasimply by changing the type of polymers and organic linkers we use, or byadjusting the amount of heat we apply during the fabrication process,"says graduate student and co-lead author of the study, John To.
Using this process, the team wasable to produce a 10-fold increase in pore volume by raising the processingtemperature from 400° C (750° F) to 900° C (1,650° F). They were also able to produce 28 g (1 oz) of carbon materialboasting a surface area of 4,073 sq m (43,840 sq ft), or the equivalent ofthree American football fields. The team says this is a new record, eclipsingconventional activated carbon, whose surface area maxes out at around 3,000sq m (32,290 sq ft).
To test the performance of the material inreal-world conditions, the team coated it on electrodes, which they installedin lithium-sulfur batteries and supercapacitors.
The researchers found the material overcamea major shortcoming of lithium-sulfur batteries. Although they boast superior energystorage capabilities to their lithium-ion counterparts, when lithium and sulfurreact, they produce lithium polysulfide at the electrode, which can leak intothe electrolyte and cause the battery to fail, resulting in significantly shorter lifetimes.
However, with the ability to tune the sizeof the pores in the designer carbon, the Stanford team was able to createelectrodes with pores big enough to let lithium ions pass through, but smallenough to trap the polysulfides, thereby improving the battery's performance and extending their life.
The material also proved beneficial for usein supercapacitors, which boast ultra-fast charging and dischargingcapabilities. Equipping supercapacitors with the designer carbon, researchers reported athree-fold improvement in electrical conductivity over electrodes made ofconventional activated carbon, while also improving the power delivery rate andstability of the electrodes.
The researchers say the surface areaattributes, the ability to fine-tune the material, and the simplicity and lowcost of the production process gives the designer carbon great potential for awide range of applications.
"High surface area isessential for many applications, including electrocatalysis, storing energy andcapturing carbon dioxide emissions from factories and power plants," Baosaid.
The team's study appears in the journal ACSCentral Science.
Source: Stanford University
